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Phosphodiesterase (which may be referred to simply as PDE, hereinafter) is an enzyme that degrades cyclic AMP (cAMP) and cyclic GMP (cGMP), which play a significant role in various reactions in cells. In response to various extracellular signals, cAMP and cGMP are generated from ATP and GTP by the action of adenylyl cyclase and guanylyl cyclase and are degraded by PDE into 5′-AMP and 5′-GMP, respectively. Eleven different families of PDE have been identified so far. Each family specifically degrades cAMP, cGMP or both and has a different tissue distribution. It is thus believed that different types of PDE control cellular reactions in different organs.
Among many PDE inhibitors that have thus far been put to practical application are those inhibiting PDE3 (treatment for heart failure), PDE4 (treatment for asthma and COPD) and PDE5 (treatment for male erectile dysfunction).
In 1999, the presence of PDE10A, a new subtype of PDE, was reported in humans, mice, and rats. PDE10A is involved in the regulation of intracellular cAMP and cGMP levels and, in humans, exists predominantly in brain, testis, and thyroid. In brain, PDE10A is predominantly expressed in putamen and caudate nucleus that form neostriatum (Non-patent Article 1). High level expression of PDE10A is observed in brain and testis of mice and rats (Non-patent Article 2).
Human PDE10A gene was isolated and was evaluated for the susceptibility to different PDE inhibitors. The results indicated that PDE10A was inhibited by dipyridamole (Patent Article 1 and Patent Article 2). No specific examples were presented of application of the compound to actual disorders.
Only one example was reported, in which minocycline was used in the patients with Huntington's disease as a PDE10A modulator with positive results (Patent Article 3).
Pyrazolo[1,5-a]pyridine derivatives as represented by the general formula (1) below are known (Patent Article 4). Of these derivatives, 3-isobutyryl-2-isopropyl pyrazolo[1,5-a]pyridine, commonly known as ibudilast, is widely used as a cerebral circulation improver or a treatment for bronchial asthma and allergic conjunctivitis. Known effects of ibudilast include enhancement of the relaxation effect of prostacyclin (PGI2) on cerebral vascular smooth muscle, enhancement of the inhibitory effect of prostacyclin (PG12) on platelet aggregation, suppression of airway contraction, suppression of leukotriene release and inhibition of PDE, as well as improvement of memory disorder (Patent Article 5) and amelioration of multiple sclerosis (Non-patent Article 3).
Although ibudilast has been described to inhibit PDE3, PDE4, and PDE5 (Non-patent Article 4), nothing has been known about its activity as a PDE10 inhibitor.